You have probably found the VPN Support page in our Wiki, which states:

Since Firmware version the VPN feature is no longer shipped with the default firmware due to security considerations. Snom provides the VPN feature in a small patch firmware which needs to be installed after the firmware has been installed.

However, you are unable to find the link for the D305. Unfortunately, the D305 does not have support for the OpenVPN enabled and can not connect to a VPN. There are currently no plans to add VPN support to the D305. If you are in need of an affordable alternative, the D315 is a suitable solution.